Diamond machining offers high accuracy and surface finish, and is suitable for fabricating optical-grade molds for making optical components, such as lenses and gratings. For example, diamond tools may be used to machine Ni molds for making gratings used in optical pickup devices. Diamond turning, fly-cutting, and vibration assisted machining (VAM) are three precision diamond machining methods that have been tried. However, each has limitations that cannot satisfy some imagined device geometry configurations for the optical pickups.
Two other diamond machining methods, slow tool servo (STS) and fast tool servo (FTS) have also been advanced as alternative precision machining methods for producing a number of these imagined device geometry configurations.
The STS technique expands the capability of a traditional diamond turning machine. It enables the creation of surface structures such as micro-prisms, torics, off-axis aspheres, and freeform optics in general. STS uses only the translation and/or rotation stages of an ultra-precision diamond turning machine to carry out all machining motion of the machining tool. Given these diamond turning machines contain powerful motion controllers, tool-path processing can be done by the machine itself. However, the available bandwidth of Z-motion (motion normal to the substrate surface) is determined by the limitations of the motion stages of diamond turning machine. Thus, the bandwidth for STS is often less than 10 Hz for the linear feed axes. Still freeform optics or diffractive patterns can be generalized and/or characterized by a function which determines machine tool position.
In FTS, a piezoelectric transduction actuator, linear motor, or voice coil actuator, drives a diamond tool in a single axis motion at high frequency. The range of motion can be 10's of microns to nearly 1 millimeter. Some systems provide 100 um of travel while maintaining 1 kHz bandwidth. The advantage of FTS is the increased bandwidth this add-on tool provides, over the traditional diamond turning machine axes. Several examples of FTS systems are available, including systems developed by the Precision Engineering Center (PEC) at the North Carolina State University and Precitech Inc.
The FTS system also expands the capability of a traditional diamond turning machine, enabling the creation of surface structures such as micro-prisms, torics, off-axis aspheres, and freeform surfaces in general. For example, using the Precitech system, the tool path software uses a C-program or bitmap image to describe the desired part topography. An external general purpose computer with a digital signal processor uses high resolution angular feedback on the work spindle and the linear position feedback of the machine translational slide to accomplish “real-time” calculations of the axial position of the tool. Both the Precitech and PEC FTS systems are designed to produce very high dynamic movement during the turning process with low mechanical noise.
A system utilizing the full capabilities of a four axis diamond turning machine, in conjunction STS or FTS may have additional capabilities that other traditional diamond turning and fly-cutting method do not allow. For example reduced tool lead-in and lead-out zones (transition zones) may be realized. Additionally, there is the potential of machining curved grooves in the work pieces using STS or FTS. These capabilities are desirable for making many types of optical components, including lenses and gratings, as well as molds for optical components that may be used in optical pickups.
Using FTS, if the linear speeds of the tool relative to the surface are similar to those used in VAM, the surface finish and cut quality may be maintained at levels comparable to those achieved using VAM. However, using either STS or FTS allows for machining with unique orientations between tool and workpiece that are not achievable with VAM, which may provides an FTS with the ability to satisfy device geometry requirements that may not be achieved with a VAM system. Tests done by the inventors have shown that optical performance of gratings may be maintained at linear speed rates of only 35 mm/min in electroless nickel.
A desired characteristic of some optical pickup designs is a minimum transition zone. It may possible to realize this with sharp tool lead-out by the coordination of diamond turning machine axes using STS or FTS. By using a full positioning diamond turning machine with either STS or FTS, the feed—controlled by the machine—may be slowed down to decrease linear surface speed. Given a fixed bandwidth and retraction time, this will effectively reduce the lead-out transition zone. Optical properties in the transition zones of these exemplary optical pickup designs may not be overly stringent. Therefore, the degradation of surface finish that may result from slowing the feed may be acceptable.
The present invention may extend the capabilities of either an STS or an FTS system by allowing for additional control of the shape of the groove machined by the precision machining system during each pass of the machining tool.